Steel-making processes

Abstract:

Steel is made by the Bessemer, Siemens Open Hearth, basic oxygen furnace, electric arc, electric high-frequency and crucible processes. In both the Acid Bessemer and Basic Bessemer (or Thomas) processes molten pig iron is refined by blowing air through it in an egg-shaped vessel, known as a converter, of 15-25 tonnes capacity. In the Siemens process, both acid and basic, the necessary heat for melting and working the charge is supplied by oil or gas.

Both the gas and air are preheated by regenerators, two on each side of the furnace, alternatively heated by the waste gases. The regenerators are chambers filled with checker brickwork, brick and space alternating. The high nitrogen content of Bessemer steel is a disadvantage for certain cold forming applications and continental works have, in recent years, developed modified processes in which oxygen replaces air.

Steel is made by the Bessemer, Siemens Open Hearth, basic oxygen furnace, electric arc, electric high-frequency and crucible processes.

Crucible and high-frequency methods

The Huntsman crucible process has been superseded by the high frequency induction furnace in which the heat is generated in the metal itself by eddy currents induced by a magnetic field set up by an alternating current, which passes round water-cooled coils surrounding the crucible. The eddy currents increase with the square of the frequency, and an input current which alternates from 500 to 2000 hertz is necessary. As the frequency increases, the eddy currents tend to travel nearer and nearer the surface of a charge (i.e. shallow penetration). The heat developed in the charge depends on the cross-sectional area which carries current, and large furnaces use frequencies low enough to get adequate current penetration.

Automatic circulation of the melt in a vertical direction, due to eddy currents, promotes uniformity of analysis. Contamination by furnace gases is obviated and charges from 1 to 5 tonnes can be melted with resultant economy. Consequently, these electric furnaces are being used to produce high quality steels, such as ball bearing, stainless, magnet, die and tool steels.

Figure 1. Furnaces used for making pig iron and steels. RH side of open hearth furnace

shows use of oil instead of gas

Acid and basic steels

The remaining methods for making steel do so by removing impurities from pig iron or a mixture of pig iron and steel scrap. The impurities removed, however, depend on whether an acid (siliceous) or basic (limey) slag is used. An acid slag necessitates the use of an acid furnace lining (silica); a basic slag, a basic lining of magnesite or dolomite, with line in the charge. With an acid slag silicon, manganese and carbon only are removed by oxidation, consequently the raw material must not contain phosphorus and sulphur in amounts exceeding those permissible in the finished steel.

In the basic processes, silicon, manganese, carbon, phosphorus and sulphur can be removed from the charge, but normally the raw material contains low silicon and high phosphorus contents. To remove the phosphorus the bath of metal must be oxidised to a greater extent than in the corresponding acid process, and the final quality of the steel depends very largely on the degree of this oxidation, before deoxidisers-ferro-manganese, ferro-silicon, aluminium-remove the soluble iron oxide and form other insoluble oxides, which produce non-metallic inclusions if they are not removed from the melt:

2Al + 3FeO (soluble) 3Fe + Al2O3 (solid)

In the acid processes, deoxidation can take place in the furnaces, leaving a reasonable time for the inclusions to rise into the slag and so be removed before casting. Whereas in the basic furnaces, deoxidation is rarely carried out in the presence of the slag, otherwise phosphorus would return to the metal. Deoxidation of the metal frequently takes place in the ladle, leaving only a short time for the deoxidation products to be removed. For these reasons acid steel is considered better than basic for certain purposes, such as large forging ingots and ball bearing steel. The introduction of vacuum degassing hastened the decline of the acid processes.

Bessemer steel

In both the Acid Bessemer and Basic Bessemer (or Thomas) processes molten pig iron is refined by blowing air through it in an egg-shaped vessel, known as a converter, of 15-25 tonnes capacity (Fig. 1). The oxidation of the impurities raises the charge to a suitable temperature; which is therefore dependent on the composition of the raw material for its heat: 2% silicon in the acid and 1,5-2% phosphorus in the basic process is normally necessary to supply the heat. The "blowing" of the charge, which causes an intense flame at the mouth of the converter, takes about 25 minutes and such a short interval makes exact control of the process a little difficult.

The Acid Bessemer suffered a decline in favour of the Acid Open Hearth steel process, mainly due to economic factors which in turn has been ousted by the basic electric arc furnace coupled with vacuum degassing.

The Basic Bessemer process is used a great deal on the Continent for making, from a very suitable pig iron, a cheap class of steel, e.g. ship plates, structural sections. For making steel castings a modification known as a Tropenas converter is used, in which the air impinges on the surface of the metal from side tuyeres instead of from the bottom. The raw material is usually melted in a cupola and weighed amounts charged into the converter.

Open-hearth processes

In the Siemens process, both acid and basic, the necessary heat for melting and working the charge is supplied by oil or gas. But the gas and air are preheated by regenerators, two on each side of the furnace, alternatively heated by the waste gases. The regenerators are chambers filled with checker brickwork, brick and space alternating.

The furnaces have a saucer-like hearth, with a capacity which varies from 600 tonnes for fixed, to 200 tonnes for tilting furnaces (Fig. 1). The raw materials consist essentially of pig iron (cold or molten) and scrap, together with lime in the basic process. To promote the oxidation of the impurities iron ore is charged into the melt although increasing use is being made of oxygen lancing. The time for working a charge varies from about 6 to 14 hours, and control is therefore much easier than in the case of the Bessemer process.

The Basic Open Hearth process was used for the bulk of the cheaper grades of steel, but there is a growing tendency to replace the OH furnace by large arc furnaces using a single slag process especially for melting scrap and coupled with vacuum degassing in some cases.

Electric arc process

The heat required in this process is generated by electric arcs struck between carbon electrodes and the metal bath (Fig. 1). Usually, a charge of graded steel scrap is melted under an oxidising basic slag to remove the phosphorus. The impure slag is removed by tilting the furnace. A second limey slag is used to remove sulphur and to deoxidise the metal in the furnace. This results in a high degree of purification and high quality steel can be made, so long as gas absorption due to excessively high temperatures is avoided. This process is used extensively for making highly alloyed steel such as stainless, heat-resisting and high-speed steels.

Oxygen lancing is often used for removing carbon in the presence of chromium and enables scrap stainless steel to be used. The nitrogen content of steels made by the Bessemer and electric arc processes is about 0,01-0,25% compared with about 0,002-0,008% in open hearth steels.

Oxygen processes

The high nitrogen content of Bessemer steel is a disadvantage for certain cold forming applications and continental works have, in recent years, developed modified processes in which oxygen replaces air. In Austria the LID process (Linz-Donawitz) converts low phosphorus pig iron into steel by top blowing with an oxygen lance using a basic lined vessel (Fig. 2b). To avoid excessive heat scrap or ore is added. High quality steel is produced with low hydrogen and nitrogen (0,002%). A further modification of the process is to add lime powder to the oxygen jet (OLP process) when higher phosphorus pig is used.

Figure 2.

The Kaldo (Swedish) process uses top blowing with oxygen together with a basic lined rotating (30 rev/min) furnace to get efficient mixing (Fig. 2a). The use of oxygen allows the simultaneous removal of carbon and phosphorus from the (P, 1,85%) pig iron. Lime and ore are added. The German Rotor process uses a rotary furnace with two oxygen nozzles, one in the metal and one above it (Fig. 2c). The use of oxygen with steam (to reduce the temperature) in the traditional basic Bessemer process is also now widely used to produce low nitrogen steel. These new techniques produce steel with low percentages of N, S, P, which are quite competitive with open hearth quality.

Other processes which are developing are the Fuel-oxygen-scrap, FOS process, and spray steelmaking which consists in pouring iron through a ring, the periphery of which is provided with jets through which oxygen and fluxes are blown in such a way as to "atomise" the iron, the large surface to mass ratio provided in this way giving extremely rapid chemical refining and conversion to steel.

Vacuum degassing is also gaining ground for special alloys. Some 14 processes can be grouped as stream, ladle, mould and circulation (e.g. DH and RH) degassing methods, Fig. 3. The vacuum largely removes hydrogen, atmospheric and volatile impurities (Sn, Cu, Pb, Sb), reduces metal oxides by the C – O reaction and eliminates the oxides from normal deoxidisers and allows control of alloy composition to close limits. The clean metal produced is of a consistent high quality, with good properties in the transverse direction of rolled products. Bearing steels have greatly improved fatigue life and stainless steels can be made to lower carbon contents.

Figure 3. Methods of degassing molten steel

Vacuum melting and ESR. The aircraft designer has continually called for new alloy steels of greater uniformity and reproducibility of properties with lower oxygen and

sulphur contents. Complex alloy steels have a greater tendency to macro-segregation, and considerable difficulty exists in minimising the non-metallic inclusions and in accurately controlling the analysis of reactive elements such as Ti, Al, B. This problem led to the use of three processes of melting.

(a) Vacuum induction melting within a tank for producing super alloys (Ni and Co base), in some cases for further remelting for investment casting. Pure materials are used and volatile tramp elements can be removed. (b) Consumable electrode vacuum arc re-melting process (Fig. 4) originally used for titanium, was found to eliminate hydrogen, the A and V segregates and also the large silicate inclusions. This is due to the mode of solidification. The moving parts in aircraft engines are made by this process, due to the need for high strength cleanness, uniformity of properties, toughness and freedom from hydrogen and tramp elements. (c) Electroslag refining (ESR) This process, which is a larger form of the original welding process, re-melts a preformed electrode of alloy into a water-cooled crucible, utilising the electrical resistance heating in a molten slag pool for the heat source (Fig. 5). The layer of slag around the ingot maintains vertical unidirectional freezing from the base. Tramp elements are not removed and lead may be picked up from the slag.

Figure 4. Typical vacuum arc remelting

furnace

Figure 5.Electroslag remelting furnace

Clean Steel: Part One

Abstract:

Steel cleanliness is an important factor of steel quality and the demand for cleaner steels increases every year. The so-called clean steel generally is the steel in which the content of impurity elements, such as phosphorus, sulphur, total oxygen, nitrogen, hydrogen (including carbon sometimes) and inclusions are very low. The improvement of steel cleanliness has therefore become a more and more important subject in the development of ferrous metallurgical technology, and also an important task for the iron and steel producers.

Steel cleanliness is an important factor of steel quality and the demand for cleaner steels increases every year. The so-called clean steel generally is the steel in which the content of impurity elements, such as phosphorus, sulphur, total oxygen, nitrogen, hydrogen (including carbon sometimes) and inclusions are very low. The improvement of steel cleanliness has therefore become a more and more important subject in the development of ferrous metallurgical technology, and also an important task for the iron and steel producers.

The demand for better mechanical properties of steels was urging steel producers to improve cleanliness of their final products. In order to obtain the satisfactory cleanliness of steel it is necessary to control and improve a wide range of operating practices throughout the steelmaking processes like deoxidant- and alloy additions, secondary metallurgy treatments, shrouding systems and casting practice.

Due to the vague nature of the term "clean steel", some authors imply that it is more precise to refer to:

steels with low levels of solutes as "high purity steels" steels with low levels of impurities that originate from the re-melting scrap as "low residual steels"

steels with a low frequency of product defects that can be related to the presence oxides as "clean steels".

It has been well known that the individual or combined effect of carbon [C], phosphorus [P], sulphur [S], nitrogen [N], hydrogen [H] and total oxygen (T.O.) in steel can have a remarkable influence on steel properties, such as tensile strength, formability, toughness, weldability, cracking-resistance, corrosion-resistance, fatigue-resistance, etc. Also, clean steel requires control of non-metallic oxide inclusions and controlling their size distribution, morphology and composition.

The control of the elements mentioned above is different for different performance demands. Those impurity elements also vary with different grades of steel. Table 1 lists the influence of common steel impurities on steel mechanical properties which means that some element is harmful to certain steel grades, but may be less harmful or even useful to another steel grades.

For examples for IF steels, the content of carbon, nitrogen, total oxygen and inclusions should be as low as possible in order to get good flexibility, high "r" value, perfect surface quality etc. In other hands the high quality

pipeline steel requires ultra low sulphure, low phosphorus, low nitrogen, low total oxygen content and a certain ratio of Ca/S.

Element Form Mechanical Properties Affected

S, O Sulfide and oxide inclusions

Ductility, Charpy impact value, anisotropy Formability (elongation, reduction of area and bendability) Cold forgeability, drawability Low temperature toughness Fatigue strength

C, N

Solid solution Solid solubility (enhanced), hardenability

Settled dislocation Strain aging (enhanced), ductility and toughness (lowered)

Pearlite and cementite Dispersion (enhanced), ductility and toughness (lowered)

Carbide and nitride precipitates Precipitation, grain refining (enhanced), toughness (enhanced) Embrittlement by intergranular precipitation

P Solid solution Solid solubility (enhanced), hardenability (enhanced) Temper brittleness Separation, secondary work embrittlement

Table 1: Influence of typical impurities on mechanical properties

As we mentioned before, steel cleanliness depends on the amount, morphology and size distribution of non-metallic inclusions. The inclusions generate many defects and many applications restrict the maximum size of inclusions so the size distribution of inclusions in steel products is also important. For certain applications where stringent mechanical properties are required the internal cleanliness of steel is very important. Table 2 shows the cleanliness requirements for various steel grades.

Steel product Maximum allowed impurity fractionMaximum allowed inclusion

size

IF steels[C]≤30 ppm, [N]≤40 ppm, T.O.≤40

ppm [C]≤10 ppm, [N]≤50 ppm

 

Automotive and deep-drawing Sheets

[C]≤30 ppm, [N]≤30 ppm 100 µm

Drawn and Ironed cans[C]≤30 ppm, [N]≤40 ppm, T.O.≤20

ppm20 µm

Alloy steel for Pressure vessels [P]≤70 ppm  

Alloy steel bars [H]≤2 ppm, [N]≤20 ppm, T.O.≤10 ppm  

HIC resistant steel sour gas tubes [P]≤50 ppm, [S] ≤10 ppm  

Line pipes[S]≤30 ppm, [N]≤50 ppm, T.O.≤30

ppm100 µm

Sheets for continuous annealing [N]≤20 ppm  

Plates for welding [H]≤1.5 ppm  

Bearings T.O.≤10 ppm 15 µm

Tire cord [H]≤2 ppm, [N]≤40 ppm, T.O.≤15 ppm 10 µm

Non-grain-orientated Magnetic Sheets

[N]≤30 ppm  

Heavy plate steels[H]≤2 ppm, [N]=30-40 ppm, T.O.≤20

ppmSingle inclusion 13 µm

Cluster 200 µm

Wires [N]≤60 ppm, T.O.≤30 ppm 20 µm

Table 2: Cleanliness requirements for various steel grades

As Table 2 shows for sheets used for car body, carbon [C], nitrogen [N], and total oxygen (T.O.) are each required to be very low. For sheets for tin plate application, total oxygen is not only needed below 20 ppm, but the size of the non-metallic inclusions in steel has to be less than 20 µm.

For steel cord used in tires, the size of non-metallic inclusions in steel has to be less than 10 μm and even smaller (5 µm) for TV shadow masks. For ball bearings, in order to improve their fatigue-resistance properties, T.O. in steel has to be below 10 ppm and the size of non-metallic inclusions has to be less than 15 µm. For meeting the specification of increasingly improved toughness for petroleum pipeline and of Hydrogen Induced Cracking (HIC) resistance for the transport of sour natural gas, the sulphur [S] content in steel has to be extremely low, less than 10 ppm.

Steel cleanliness is controlled by a wide range operating practices throughout the steelmaking processes. These include the time and location of deoxidant and alloy additions, the extent and sequence of secondary metallurgy treatments, stirring and transfer operations, shrouding systems, tundish geometry and practices, the absorption capacity of the various metallurgical fluxes, and casting practices.

A one of the steelmaking process routes for the production of clean steels is outlined in Figure 1.

Figure 1: The process route for the production of clean steels

Clean Steel: Part Two

Abstract:

Non-metallic inclusions, which are undesirable components of all steels, play an important role with respect to their effect on the steel properties. Controlling inclusions in steel is closely connected with the concept of "clean steel". The improvement in steel properties by control of non-metallic inclusions plays an important part in defending the applications of steel against newer competitive materials.

Non-metallic inclusions, which are undesirable components of all steels, play an important role with respect to their effect on the steel properties. Controlling inclusions in steel is closely connected with the concept of “clean steel”. The improvement in steel properties by control of non-metallic inclusions plays an important part in defending the applications of steel against newer competitive materials. The aims of the metallurgist are to eliminate undesirable inclusions and control the nature and distribution of the remainder to optimize the properties of the final product.

Generally, non-metallic inclusions in steel normally have a negative contribution to the mechanical properties of steel, since they can initiate ductile and brittle facture. Among various types of nonmetallic inclusions, oxide and sulphide inclusions have been thought harmful for common steels.

All steels contain non-metallic inclusions to a greater or less extent. The type and appearance of these non-metallic inclusions depends on factors such as grade of steel, melting process, secondary metallurgy treatments and casting of steel. Because of this, it is of particular significance to determine how pure the steel is. The term steel cleanness is relative one, since even steel with only 1 ppm each of oxygen and sulfide will still contains 109 -1012 non-metallic inclusions per ton. From the viewpoint of “cleanness” all steels are “dirty”.

Non metallic inclusions in steel are the cause for dangerous and serious material defects such as brittleness and a vide variety of crack formations. However, some of these inclusions can also have a beneficial effect on steels properties by nucleating acicular

ferrite during the austenite to ferrite phase transformation especially in low carbon steels. According to definition, the non-metallic inclusions are chemical compounds of metal with nonmetal which are present in steel and alloys like separated parts.

Classification of non-metallic inclusionsNon-metallic inclusions are divided by chemical and mineralogical content, by stableness/stability and origin. By chemical content non-metallic inclusions are divided into the following groups:

Oxides (simple: FeO, MnO, Cr2O3, TiO2, SiO2, Al2O3 etc.; compound: FeOFe2O3, FeOAl2O3, MgOAl2O3, FeOCr Sulphides (FeS, MnS, CaS, MgS, Al2S3 etc.; compound: FeSFeO, MnSMnO etc.) Nitrides (simple: TiN, AlN, ZrN, CeN etc.; compound: Nb(C,N), V(C,N) etc, which can be found in alloyed steels and has

strong nitride-generative elements in its content: titanium, aluminum, vanadium, cerium etc.) Phosphides (Fe3P, Fe2P etc.)

The majority of inclusions in steels are oxides and sulphides. Among various types of nonmetallic inclusions, oxide and sulphide inclusions have been thought harmful for common steels. Usually, nitrides are present in special steels (stainless steels, tool steels) which have elements with a strong affinity for nitrogen (e.g. chrome, vanadium), which create nitrides.

Figure 1 shows sulfides and oxides of non metallic inclusion in steel.

Figure 1: Non-metallic inclusion in steel: oxides-dark gray and sulfides-light gray

By mineralogical content oxygen inclusions are divided into the following groups:

Free oxides – FeO, MnO, Cr2O3, SiO2 (quartz), Al2O3 (corundum) etc. Spinels-compound oxides which are formed by bi- and tri-valent elements as a ferrites, chromites and aluminates. Silicates which are presented in steel like a glass formed with pure SiO2 or SiO2 with admixture of iron, manganese,

chromium, aluminum and tungsten oxides and also crystalline silicates.

Depending on the melting temperature, in liquid steel non-metallic inclusions are in solid or liquid condition.

As mentioned above the majority of inclusions in steels are oxides and sulfides. Sulfides in steel have been paid much attention because their treatment is an important problem in the steelmaking process. They affect on the properties of the final products by their deformation during the steel working process; especially their morphology has a significant effect on the steel properties.

According to analysis based on the steel ingots containing 0.01-0.15% S, the morphology of MnS can be classified into three types:

1) Type I is a globular .MnS with a wide range of sizes, and is often duplex with oxides.2) Type II has a dendritic structure and is often called grain-boundary sulfide because it is distributed as chain-like formation or thin precipitates in primary ingot grain boundaries.

3) Type III is angular sulfide and always forms as monophase inclusion.

Most of the above mentioned sulfides are formed both during the process of secondary metallurgy or the solidification process. Recently, with the development of steelmaking technology, the sulfur concentration in steel was lowered drastically. Also, the continuous casting technology of steels with higher cooling rate than the ingot casting almost replaced the ingot casting.

So, the sulfides in the modern commercial steel are usually formed on solidification process or in solid steel during the subsequent cooling process. For example, the Widmanstätten plate-like MnS2, is formed in solid steel and Figure 2 shows the common morphology of MnS in conventional continuously casting steel, including the globular duplex oxide–sulfide (particle A, B and C) and the Widmanstätten plate-like MnS (particle D).

Figure 2: Typical duplex oxide–sulfide inclusion (particle A, B and C) and plate-like MnS (particle D) in conventional continuous casting silicon steel.

Numerous examples of the effect of non-metallic inclusions on steel properties show the importance of the behavior of the inclusions as well as of surrounding metal matrix during plastic working of steels. The aims of the metallurgist are to eliminate undesirable inclusions and control the nature and distribution of the remainder to optimize the properties of the final product.

An attempt by using program ABACUS was performed to model the behavior of slag inclusions and their surrounding matrix material during hot rolling and hot forging of hardenable steels. It is shown that it can be helpful for studying the behavior of inclusions, which is difficult or even impossible to obtain from a conventional experiment.

Figure 3 shows the effective strain contour during plastic deformation. Three regions of strain concentration (red) can be seen and a trihedral void (white region) close to the round inclusion is formed. The strain concentrations arise at the inner surface of the matrix. Another interested thing is that two edges of the pore tend to emerge and a bonding is formed. The difference in mechanical properties between the matrix and the inclusion is found to be the primary reason to create a void. The weak bonding at the interface between the matrix and the inclusion seems to facilitate to open the void.

Figure 4 shows the effect of rolling temperature on the relative plasticity index during hot rolling of steels. The relative plasticity index of inclusion increases while the rolling temperature rises. There exists a transition region, where the relative plasticity index changes rapidly. This trend agrees with the existing experimental results.

Figure 3: Void formation close to the inclusion.

Figure 4: Effects of rolling temperature on the relative plasticity index.

Clean Steel: Part Three

Abstract:

The presence of non-metallic oxide inclusions is a major cause of incompatibility between the attainable and desirable level of cleanliness in many grades of commercial steel. Generally, inclusions degrade the mechanical properties of the steel and thereby reduce the ductility of the cast metal and increase the risk for mechanical and/or corrosion failure of the final product.

The increasing demand in recent years for high-quality steel products has led to the continuous improvement of steelmaking practices. There is a special interest in the control of non-metallic inclusions due to their harmful effect on the subsequent stages and their great influence on the properties of the final product. Through the control of the amount, size and chemical composition of the inclusions it is possible to obtain a final product of good quality. The control of the formation of non-metallic inclusions and the identification of their constituent phases are of extreme importance for the obtaining of clean steels.

The presence of non-metallic oxide inclusions is a major cause of incompatibility between the attainable and desirable level of cleanliness in many grades of commercial steel. Generally, inclusions degrade the mechanical properties of the steel and thereby reduce the ductility of the cast metal and increase the risk for mechanical and/or corrosion failure of the final product.

Oxide inclusions originate from two sources:

residual products resulting from intentionally added alloying elements to deoxidize the molten steel after oxygen treatment (endogenous or micro inclusions);

products resulting from reactions between the melt and atmosphere, slag, or refractory (exogenous or macro inclusions).

Among various types of nonmetallic inclusions, oxide and sulphide inclusions have been thought harmful for common steels.

Alumina inclusions occur as deoxidation products in the aluminum-based deoxidation of steel. Pure alumina has a melting point above 2000°C, i.e., these alumina inclusions are present in a solid state in liquid steel. The addition of calcium to steel which contains such inclusions changes the composition of these inclusions from pure alumina to CaO-containing calcium aluminates.

As it can be see from Figure 1, the, melting point of the calcium aluminates will decrease as the CaO content increases, until liquid oxide phases occur at about 22% of CaO, i.e., when the CaO.2Al2O3 compound is first exceeded at 1600°C. The liquid phase content continues to increase as CaO content rises further and is 100% at 35% of CaO. The minimum melting temperature for the liquid calcium aluminates is around 1400°C, i.e., such liquid calcium aluminates may be present in liquid form until, or even after, the steel solidifies.

Most grades of steel are treated with calcium using either a Ca-Si alloy or a Ca-Fe(Ni) mixture, depending on the silicon specification. This treatment is made after trim additions and argon rinsing.

In most melt shops the cored wire containing Ca-Si or Ca-Fe(Ni) injection system is used in the calcium treatment of steel. The melting and boiling points of calcium are 839°C and 1500°C respectively. During calcium treatment, the alumina and silica inclusions are converted to molten calcium aluminates and silicate which are globular in shape because of the surface tension effect. The change in inclusion composition and shape is known as the inclusion morphology control.

Figure 1: Binary system CaO-Al2O3

The calcium aluminates inclusions retained in liquid steel suppress the formation of MnS stringers during solidification of steel. This change in the composition and mode of precipitation of sulphide inclusion during solidification of steel is known as sulphide morphology or sulphide shape control.

Several metallurgical advantages are brought about with the modification of composition and morphology of oxide and sulphide inclusions by calcium treatment of steel, as for instance:

To improve steel castability in continuous casting, i.e. minimize nozzle blockage To minimize inclusion related surface defects in billet, bloom and slab castings To improve steel machinability at high cutting speeds and prolong the carbide tool life To minimize the susceptibility of steel to re-heat cracking, as in the heat-affected zones (HAZ) of welds To prevent lamellar tearing in large restrained welded structures To minimize the susceptibility of high-strength low alloy (HSLA) linepipe steels to hydrogen-induced cracking (HIC) in

sour gas or sour oil environments. The Ca content in the final product can be controlled within the range of 15 to 20 ppm To increase both tensile ductility and impact energy in the transverse and through-thickness directions in steels with

tensile strengths below 1400 MPa

When calcium is injected deep into the melt, the following series of reactions are expected to occur to varying extents in Al-killed steels containing alumina inclusions:

Ca + O = CaO       (1)

Ca + S = CaS       (2)

Ca + (x+1/3)Al2O3 = CaO·x Al2O3 + 2/3[Al]       (3)

Depending on the steel composition, the manner of calcium adding in steel bath and other process variables, there will be variations in the conversion of alumina inclusions to aluminates inclusions, the smaller inclusions will be converted to molten calcium aluminates more readily than the larger inclusions.

Thermodynamically, if sulfur or oxygen is dissolved in the steel at moderate levels, or if Al2O3 inclusions are present in steel, calcium will react with oxygen or sulfur until the contents of reactants are very low (< 2ppm). One of the critical questions is whether or not calcium added to steel will react with sulfur by reaction (2) and form CaS or modify Al2O3 to liquid calcium aluminates by reaction (3).

The formation of calcium sulfide can occur if calcium and sulfur contents are sufficiently high. Since calcium has higher affinity for oxygen than for sulfur, the addition of calcium initially results in a more or less pronounced conversion of the alumina into calcium aluminates until the formation of calcium sulfides starts as the addition of calcium continues.

Calcium sulfides are solid at steelmaking temperatures and result in nozzle clogging similar to that caused by alumina. As can be observed from the Figure 2, the conversion of alumina into calcium aluminates occurs until all the inclusions in the steel are present only in liquid form.

Figure 2: Change of inclusions composition during calcium additions

To prevent nozzle clogging in continuous casting by solid inclusions, calcium is added to steel to modify inclusions and desulfurize the steel. Calcium will convert solid alumina (Al2O3) inclusions into lower melting point calcium aluminates, which will help prevent the clogging of the casting nozzles. However, when calcium is added to steel, it will also react with oxygen and sulfur and modify the sulfide inclusions. If the sulfur content of the steel is high, calcium will react with sulfur forming solid CaS, which could clog up the continuous casting nozzle.

The Figure 3 shows influence of calcium treatment on the type of inclusions formed and its relationship with nozzle clogging.

Figure 3: Influence of calcium treatment on the type of inclusions formed and its relationship with nozzle clogging

Calcium treatment cannot be applied to all kinds of steel. For those with high requirement on formability, such as automobile sheet, calcium treatment is not suitable, because this treatment causes the formation of calcium aluminates inclusion which is hard. Therefore, for those kinds of steel, the method of improving molten steel´s purity is usually taken to optimize castability. Through controlling carry-over slag from melting furnace, deformation treatment of ladle slag, metallurgy in tundish, protective casting and other measures, purity of steel is guaranteed and total oxygen content in molten steel decreases.

Nitrogen in Steels: Part One

Abstract:

All steels contain some nitrogen which is effective in improving the mechanical and corrosion properties of steels if it remains in solid solution or precipitates as very fine and coherent nitrides. When nitrogen is added to austenitic steels it can simultaneously improve fatigue life, strength, work hardening rate, wear and localized corrosion resistance.High nitrogen martensitic stainless steels show improved resistance to localized corrosion (pitting, crevice and intergranular corrosion) over their carbon containing counterparts. Because of this, the high nitrogen steels are being considered a new promising class of engineering materials.

All steels contain some nitrogen which is effective in improving the mechanical and corrosion properties of steels if it remains in solid solution or precipitates as very fine and coherent nitrides. When nitrogen is added to austenitic steels it can simultaneously improve fatigue life, strength, work hardening rate, wear and localized corrosion resistance.

High nitrogen martensitic stainless steels show improved resistance to localized corrosion (pitting, crevice and intergranular corrosion) over their carbon containing counterparts. Because of this, the high nitrogen steels are being considered a new promising class of engineering materials.

Solubility of Nitrogen

The nitrogen solubility data are summarized by the following equations and are shown graphically in Fig.1

½N2=[N]        (1)

The equilibrium constant for nitrogen absorption is therefore:

K=[ppmN] / (pN2)½        (2)

And reaction constant of this equation is related to the free energy;

       (3)

which becomes zero in case of equilibrium leading to:

       (4)

If it is assume that the activity of the nitrogen dissolved in the steel is approximately the same as the chemical concentration (Henry's law), then this concentration can be calculated as:

[%N]=KN(pN2)½        (5)

Sources of Nitrogen in Steelmaking

Numerous sources of nitrogen exist during the melting, the ladle processing and the casting operations. Sources of nitrogen in oxygen steelmaking include the hot metal, the scrap, the impurity nitrogen in oxygen and the nitrogen used as a stirring gas.

Nitrogen pickup from the atmosphere can occur during reblows in which case the furnace fills up with air, which is then entrained into the metal when the oxygen blow restarts. Also during the tapping of steel, air bubbles are entrained into the steel where the tap stream enters the bath in the ladle. Other sources may include atmosphere (through ladle slag), coke (carburizers) and various ferro-alloys. Ladle additions often contain moisture. The otherwise, to get an impression of the sources of nitrogen during the melting process, Table 1 shows the amount of nitrogen present in each of the feed materials typically used in the EAF.

Table 1: Nitrogen content of feed materials used in EAF steelmakingFeed Material Nitrogen Content

Scrap 30-120 ppm

HBI/DRI 20-30 ppm

Liquid iron from the BF 60 ppm

Cold pig iron (CPI) 20-30 ppm

Hot heel 10 ppm

Coke 5000-10000 ppm

Oxygen 30-200 ppm

Carbon Carrier Gas (Air) 78%

Bottom stirring gas(N2) > 99,9%

Bottom stirring gas (Ar) < 30 ppm

CaO 400 ppm

Behavior of Nitrogen in Steel

While steel is liquid the nitrogen present exists in the solution. However, solidification of steel may result in three nitrogen-related phenomena: formation of blowholes; precipitation of one or more nitride compounds; and/or the solidification of nitrogen

in interstitial solid solution.

The maximum solubility of nitrogen in liquid iron is approximately 450 ppm, and less than 10 ppm at ambient temperature, as shown in Figure 1. The presence of significant quantities of other elements in liquid iron affects the solubility of nitrogen. More importantly, the presence of dissolved sulfur and oxygen limit the absorption of nitrogen because they are surface-active elements. This is exploited during steelmaking to avoid excessive nitrogen pickup, particularly during tapping.

Figure 1: Solubility of nitrogen in iron for temperatures of 600-2000°C

Effect of Nitrogen on Steel Properties

The effect of nitrogen on steel properties can be either detrimental or beneficial, depending on the other alloying elements present, the form and quantity of nitrogen present, and the required behavior of the particular steel product.

In general, however, most steel products require that nitrogen be kept to a minimum. High nitrogen content may result in inconsistent mechanical properties in hot-rolled products, embrittlement of the heat affected zone (HAZ) of welded steels, and poor cold formability. In particular, nitrogen can result in strain ageing and reduced ductility of cold-rolled and annealed LCAK steels.

Effect of Nitrogen on Formability

Figure 2 shows that the strength of LCAK steels decreases slightly and then increases with increasing nitrogen. Conversely, the elongation decreases and the r-value increases with increasing nitrogen. The r-value is the average ratio of the width to thickness strain of strip tensile specimens tested in various orientations. It is an inverse measure of formability. Hence, high nitrogen content leads to poor formability of LCAK steels, even after annealing.

Figure 2: Effect of nitrogen on yield strength, tensile strength, r-value and elongation of LCAK steel in the annealed condition

The effect of nitrogen on mechanical properties is the result of interstitial solid solution strengthening by the free nitrogen; precipitation strengthening by aluminum and other nitrides; and grain refinement due to the presence of nitride precipitates.

Effect of Nitrogen on Hardness

Hardness is the resistance of a material to surface indentation. The Figure 3 shows that hardness increases linearly with increasing nitrogen content. Nitrogen absorbed during steelmaking results in interstitial solid solution strengthening and grain refinement, both of which increase hardness. Further, the diagram shows that nitrogen absorbed during the steelmaking process has a more significant impact than that absorbed during batch annealing in a nitrogen-rich atmosphere, although both have a measurable effect.

Figure 3: Increase in hardness of aluminum-killed sheet steel with respect to increasing nitrogen content

Effect of Nitrogen on Strain Ageing

Strain ageing occurs in steels containing interstitial atoms, predominantly nitrogen, after they have been plastically deformed. After deformation, the nitrogen segregates to dislocations causing discontinuous yielding when further deformed. Not only does strain ageing result in increased hardness and strength, and reduced ductility and toughness, but it may also result in the appearance of 'fluting' or 'stretcher strains' on the surface of deformed material. Duckworth and Baird have developed a measure of strain ageing termed 'strain ageing index'. This is based on an empirical equation to calculate the increase in yield stress when deformed material is held for 10 days at room temperature. Figure 4 shows that increasing nitrogen results in a higher stain-ageing index, and therefore greater propensity for surface defects.

Figure 4: Effect of nitrogen on strain ageing in mild steels with varying manganese content

Effect of Nitrogen on Impact Properties Including Welded Material

The ability of a material to withstand impact loading is commonly known as toughness. It is sometimes quantified by measuring the amount of energy that is absorbed by a test piece of known dimensions prior to fracture. It is further analyzed by determining the fracture mechanism upon impact over a range of temperatures. As temperature is decreased, the fracture type will change from fibrous/ductile to crystalline/brittle. This arbitrary temperature is termed the 'ductile-to-brittle' transition temperature. The lower the transition temperature the better the impact properties, since failure via ductile fracture may be less catastrophic than that via brittle failure. Figure 5 demonstrates that as free nitrogen increases, the transition temperature increases, and therefore toughness decreases. This is attributed to solid solution strengthening.

Figure 5: Effect of free nitrogen on impact properties

Conversely, limited amounts of nitrogen present as precipitates have a beneficial effect on impact properties. Nitrides of aluminum, vanadium, niobium and titanium result in the formation of fine-grained ferrite. Further, the smaller the grain size the lower the transition temperature, hence improved toughness. Therefore, it is necessary to carefully control, not only the nitrogen content, but also the form in which it exists, in order to optimize impact properties. Nitrogen is known to affect the toughness of the heat-affected zone (HAZ) of welded steel. This is important, since the weld metal should not be a point of weakness in a welded structure. This loss in toughness is often referred to as HAZ embrittlement. It is thought this occurs when the nitrides present in the HAZ are dissociated as a result of the elevated temperatures that exist during welding. The absence of precipitates results in grains of larger diameter. Also, the metal cools quickly producing low toughness martensite or bainite, which contain high levels of free nitrogen further exacerbating the loss of toughness. Using lower heat input and several passes to prevent dissociation of the nitrides may prevent this.

Nitrogen in Steels: Part Two

Abstract:

The effect of nitrogen on mechanical properties is the result of interstitial solid solution strengthening by the free nitrogen; precipitation strengthening by aluminum and other nitrides; and grain refinement due to the presence of nitride precipitates.Nitrogen absorbed during steelmaking results in interstitial solid solution strengthening and grain refinement,

both of which increase hardness.

Effect of Nitrogen on Formability

Figure 1 shows that the strength of LCAK steels decreases slightly and then increases with increasing nitrogen. Conversely, the elongation decreases and the r-value increases with increasing nitrogen. The r-value is the average ratio of the width to thickness strain of strip tensile specimens tested in various orientations. It is an inverse measure of formability. Hence, high nitrogen content leads to poor formability of LCAK steels, even after annealing.

Figure 1: Effect of nitrogen on yield strength, tensile strength, r-value and elongation of LCAK steel in the annealed condition

The effect of nitrogen on mechanical properties is the result of interstitial solid solution strengthening by the free nitrogen; precipitation strengthening by aluminum and other nitrides; and grain refinement due to the presence of nitride precipitates.

Effect of Nitrogen on Hardness

Hardness is the resistance of a material to surface indentation. The Figure 2 shows that hardness increases linearly with increasing nitrogen content. Nitrogen absorbed during steelmaking results in interstitial solid

solution strengthening and grain refinement, both of which increase hardness. Further, the diagram shows that nitrogen absorbed during the steelmaking process has a more significant impact than that absorbed during batch annealing in a nitrogen-rich atmosphere, although both have a measurable effect.

Figure 2: Increase in hardness of aluminum-killed sheet steel with respect to increasing nitrogen content

Strain ageing occurs in steels containing interstitial atoms, predominantly nitrogen, after they have been plastically deformed. After deformation, the nitrogen segregates to dislocations causing discontinuous yielding when further deformed. Not only does strain ageing result in increased hardness and strength, and reduced ductility and toughness, but it may also result in the appearance of "fluting" or "stretcher strains" on the surface of deformed material.

Duckworth and Baird have developed a measure of strain ageing termed "strain ageing index". This is based on an empirical equation to calculate the increase in yield stress when deformed material is held for 10 days at room temperature. Figure 3 shows that increasing nitrogen results in a higher stain-ageing index, and therefore greater propensity for surface defects.

Figure 3: Effect of nitrogen on strain ageing in mild steels with varying manganese content

Effect of Nitrogen on Impact Properties Including Welded Material

The ability of a material to withstand impact loading is commonly known as toughness. It is sometimes quantified by measuring the amount of energy that is absorbed by a test piece of known dimensions prior to fracture. It is further analyzed by determining the fracture mechanism upon impact over a range of temperatures.

As temperature is decreased, the fracture type will change from fibrous/ductile to crystalline/brittle. This arbitrary temperature is termed the "ductile-to-brittle" transition temperature. The lower the transition temperature the better the impact properties, since failure via ductile fracture may be less catastrophic than that via brittle failure.

Figure 4 demonstrates that as free nitrogen increases, the transition temperature increases, and therefore toughness decreases. This is attributed to solid solution strengthening.

Figure 4: Effect of free nitrogen on impact properties

Conversely, limited amounts of nitrogen present as precipitates have a beneficial effect on impact properties. Nitrides of aluminum, vanadium, niobium and titanium result in the formation of fine-grained ferrite. Further, the smaller the grain size the lower the transition temperature, hence improved toughness. Therefore, it is necessary to carefully control, not only the nitrogen content, but also the form in which it exists, in order to optimize impact properties.

Nitrogen is known to affect the toughness of the heat-affected zone (HAZ) of welded steel. This is important, since the weld metal should not be a point of weakness in a welded structure. This loss in toughness is often referred to as HAZ embrittlement.

It is thought that this occurs when the nitrides present in the HAZ are dissociated as a result of the elevated temperatures that exist during welding. The absence of precipitates results in grains of larger diameter. Also, the metal cools quickly producing low toughness martensite or bainite, which contain high levels of free nitrogen further exacerbating the loss of toughness. Using lower heat input and several passes to prevent dissociation of the nitrides may prevent this.

The Oxygen Steelmaking Process: Part One

Abstract:

The oxygen steelmaking process is a generic name given to those processes in which gaseous oxygen is used as the primary agent for autothermic generation of heat as a result of the oxidation of dissolved impurities like carbon, silicon, manganese and phosphorus and to a limited extent the oxidation of iron itself. Several types of oxygen steelmaking processes, like top blowing, bottom blowing and combined blowing have been invented.

The oxygen steelmaking process is a generic name given to those processes in which gaseous oxygen is used as the primary agent for autothermic generation of heat as a result of the oxidation of dissolved impurities like carbon, silicon, manganese and phosphorus and to a limited extent the oxidation of iron itself. Several types of oxygen steelmaking processes, like top blowing, bottom blowing and combined blowing have been invented.

The essential features of conventional steelmaking are the partial oxidation of the carbon, silicon, phosphorus and manganese present in pig iron and the accompanying reduction in the sulfur level. Blast furnace hot metal for LD (Basic Oxygen Furnace) steelmaking process ideally contains about C=4.2%, Si=max 0.8%, Mn=max 0.8%, S=max 0.05%, P=max 0.15% but these solute elements are diluted by the addition scrap which forms some 20-30% of the metallic charge.

Refining Reactions

In LD basic oxygen steelmaking process, the oxygen required for the refining reactions is supplied as a gas and both metal and slag are initially oxidized

½O2(g) ↔ [O] .....(1)

Fe + [O] ↔ (FeO) .....(2)

2(FeO) + ½O2(g) ↔ (Fe2O3) .....(3)

Carbon

The actual distribution of oxygen between slag and metal is not easily determined since it is a function of a number of variables including lance height and oxygen flow rate. The principal refining reactions is of course the removal of carbon:

[C] + [O] ↔ CO2 .....(4)

[C] + (FeO) ↔ CO2 + Fe .....(5)

The Figure 1 represents an idealized diagram, showing the changes in concentrations of the elements in LD metal bath during oxygen blowing. The basic thermodynamic data for these reactions are well established and the equilibrium carbon and oxygen contents may be readily calculated for all the temperatures and pressures encountered in steelmaking.

Figure 1: The changes of bath composition during the blow in a basic oxygen steelmaking converter (idealized)

Oxidation of carbon during the oxygen converter process is most important, since the reaction increases the temperature and evolves a large amount of gases CO and CO2 that cause agitation of metal and slag and remove hydrogen, nitrogen and part of non-metallic inclusions from the metal. Owing to the pressure of the oxygen supplied and the evolution of large quantities of gases, the liquid bath becomes an intimate mixture of slag, metal and gas bubbles, with an enormous contact surface. Because of this, the reaction of carbon oxidation is self-accelerated and attains a very high rate.

Silicon

In accordance with thermodynamic predictions, the removal silicon is usually completed relatively early in the blow. The reaction may be represented by equations (6) and (7).

[Si] + 2[O] ↔ (SiO2) .....(6)

[Si] + 2(FeO) ↔ (SiO2) + 2Fe .....(7)

Manganese

Similar equations can be applied to manganese removal

[Mn] + [O] ↔ (MnO) .....(8)

[Mn] + (FeO) ↔ (MnO) + Fe .....(9)

Initially, the bath manganese level falls as a result of oxidation, but later, a slightly reversion, followed by a second fall occurs. These changes in the manganese content of the bath are attributed to the combined effects of rising temperature and variable slag composition, on the activities of manganese and ferrous oxides, suggesting that the reaction is close equilibrium. This view is supported by the observation that at the end of blowing, the manganese content was found to be 82% of the equilibrium value when lump lime was used and 85% of the equilibrium value when powdered lime is injected.

In the middle part of blow, the (FeO) level in the slag falls as a consequence of the decarburization process and the dilution that accompanies lime fluxing. However, towards the end of the blow, the (FeO) increases again, as carbon removal becomes less intense and dilution begins to affect the activity of manganese oxide with the result that manganese transfers from bath to slag. To some extent the manganese loss may be minimized by rising the temperature.

Phosphorus

The partitioning of phosphorus between the slag and metal is known to be very sensitive to process conditions and so far it has been possible to build a kinetic model based on simple assumptions.

The distribution of phosphorus between slag and metal has been reviewed by Healy, who concluded that the thermodynamic behavior of phosphorus is best explained by a modified version of the ionic theory first proposed by Flood and Grjotheim. The slag-metal reactions is written in ionic form in equation (10)

2[P] + 5[O] + 3 (O2-) ↔ 2(PO3-)4 .....(10)

Healy has expressed the equilibrium distribution of phosphorus by equations that apply to specific concentration ranges in the CaO-SiO2-FeO system, i.e.:

log (%P)/[P] = 22 350/T + 7 log%CaO + 2.5 log Fet – 24.0 .....(11)

log (%P)/[P] = 22 350/T + 0.08 log%CaO + 2.5 log Fet – 16.0 .....(12)

Equation (11) is applicable to slag containing over 24%CaO while equation (12) is valid from zero %CaO to saturation.

Unfortunately, in practice the phosphorus partition ratios are far from the values calculated for equilibrium with carbon-free iron, because the oxygen potential of the slag-metal system is influenced by decarburization. A limited correlation with the carbon content in the bath has been reported, although other workers have suggested that extensive dephosphorization should be possible at high carbon levels, provided that the slag is

sufficiently basic.

On the other hand, the dependence of phosphorus distribution on the FeO content of slag in LD and Q-BOP is shown in Figure 2. A parameter kPS is defined as

kPS = (%P2O5)/[%P]•(1 + (%SiO2)) = φ ((%FeO),B) .....(13)

where B is basicity; for B>2.5, kPS is found to be independent of B.

The distribution of phosphorus is also found to be related to the content of carbon in steel at the time of tapping; owing to lower carbon levels achieved in bottom-blown process, the phosphorus distribution is expected to be better than in LD. In general, high basicity and the low temperature of slag (irrespective of the FeO content) favor dephosphorization.

Figure 2: Effect of FeO content of slag on phosphorus distribution and log kPS value

Sulfur removal

Sulfur transfer takes place through the following reactions:

[S] + (O2)g = (SO2)g .....(14)

It is found that approximately 15-25% of dissolved sulfur is directly oxidized into the gaseous phase due to the

turbulent and oxidizing conditions existing in the jet impact zone.

In Basic Oxygen Furnace, the metal desulphurization proceeds slowly because it is a diffusion process. It may be speeded up by improving the bath mixing and increasing the temperature, fluidity and basicity of the slag, and the activity of sulfur. At the initial stage of the heat, when the metal is rich in carbon and silicon, the activity of sulfur is high. Besides, part of sulfur is removed at the initial stages of the process when the temperature of melt is still relatively low through its reaction with manganese:

[Mn] + [S] = (MnS) .....(15)

A rise in the concentration of iron oxides in the slag promotes dissolution of lime, and therefore favors desulphurization. But the secondary and most intensive desulphurization occurs at the end and of the heat when the lime dissolves in the slag with a maximum rate and the slag basicity reaches B=2.8 and more. Thus the total desulphurization of the metal is mainly decided by the basicity of the homogeneous final slag which is formed in the oxygen converter process during the last minutes of metal blowing.

With an increase of slag basicity, the residual concentration of sulfur in metal bath becomes lower, so that the coefficient of sulfur distribution between slag and metal can be raised up to 10. The greater the bulk of slag is the largest part of the sulfur will pass into slag at the same sulfur distribution coefficient. But it is not beneficial to form a very large bulk of slag, since this increases the iron loss due to burning, causes splashings and rapid wear of the lining.

In a steel plant, regression equations based on operational data are employed to predict the end point sulfur within acceptable limits. One such equation, for example is

(%S)/[S] = 1.42B – 0.13(%FeO) + 0.89 .....(16)

The equation 16 shows the beneficial influence of slag basicity and the retarding influence of FeO on sulfur distribution. A large number of such correlations are reported in the literature, but they are suitable and applicable to a local situation only.